Genetically diverse subtypes and recombinant forms of HIV-1 circulate in different regions of the globe and are the basis of the current AIDS pandemic. The biological consequences of this viral diversity are poorly understood and pose a substantial challenge for vaccination. In HIV-1 infected patients, the immune system makes neutralizing antibodies against the HIV-1 envelope glycoproteins, but these proteins are highly variable in sequence, especially between subtypes, and they utilize sophisticated mechanisms to avoid immune recognition. The study of virus neutralization and escape in patients that harbor globally prevalent genetic subtypes of HIV-1 is therefore of substantial interest. The goal of the proposed studies is to better understand how antibodies neutralize subtype C HIV-1, which predominates in sub-Saharan Africa, and to determine how the virus escapes from immune recognition. The studies will be performed using viral envelope clones derived from longitudinally collected material (blood cell DNA and plasma) from recently infected, treatment-naive subjects in a discordant couple cohort in Zambia, where subtype C HIV-1 predominates. The ability of antibodies in patient plasma to neutralize autologous viral envelope clones will be evaluated for a panel of 15 seroconvertors using a single round infection assay, and molecular approaches will be used to isolate patient-derived antibodies, identify neutralization targets, and reconstruct pathways of viral escape. Mutational patterns that occur in a helical domain of the envelope and are peripherally associated with neutralization escape will also be investigated. The hypotheses are that (i) the major escape pathways operative in early subtype C infection differ from those reported to dominate in subtype B infection, (ii) these escape pathways carry a fitness cost and are dependent on ancillary changes in the ?2 helix, and (iii) B cell responses in subtype C infection target strain-specific and shared regions of Env that are distinct from those defined for subtype B infection. PUBLIC HEALTH RELEVANCE: Neutralizing antibodies in HIV-1 infection are directed against the envelope (Env) glycoproteins gp120 and gp41. However, HIV-1 Env utilizes highly effective, but poorly defined, overlapping mechanisms to evade antibody-mediated neutralization. Our lack of information about the B cell responses, targets of autologous neutralization, and mechanisms of viral escape in subtype C infection represents a significant gap in our understanding of the most predominant HIV-1 variants worldwide. The goal of these studies is therefore to identify B cell targets and define mechanisms of virus neutralization and escape in newly infected subtype C patients.